Internal frame packs are generally characterized by having several various sized pocket compartments into which a metal support frame is placed. Typically, tubular sleeves are sewn onto the inside of the largest, lower compartment of the pack into which metal strips or stays are slidably placed. The stays are commonly made of high grade aluminum which is bent to the vertical contours of the wearers back. Usually the stays are attached onto a sheet of HDPE Plastic which provides a rigid surface. On the outside of this rigid surface the shoulder straps and waist belt are attached, so that the weight of the load in the pack is transferred to the waist belt of the user, and not the shoulders. An important benefit of an internal frame pack is that it fits closely to the body of the user. This close fit provides a low center of gravity, and the separate strips or stays of the internal frame allow the pack to move with the user (torsional flex). A common problem with internal frame packs is a lack of good support by the internal frame. Since the aluminum stays are designed to be bent to the contours of the wearers back, they can easily become deformed under heavy loads or if the pack is dropped. The greater the load, the more deformed the support stays become. Thus, with larger loads, weight transfer to the waist belt is often inefficient or ineffective. Other related problems with internal frame packs are the discomfort and fatigue suffered by the wearer because the frame is flimsy and insubstantial. As more is loaded into the pack, it bows away from the user's back failing to transfer the load to the waist, and instead putting more of the load on the user's shoulders. Another problem relates to the shape of the traditional internal support frame, being only contoured to the vertical contours of back, resulting in the frame being able to shift horizontally when the back is subjected to side to side movements. Finally the flexibility of the common internal support frame is consistent form top to bottom, unlike the human back which flexes differently in the lumbar region, the thoracic region and the shoulder region, causing a discomfort from either being too stiff in the lumbar region or too soft in the thoracic or shoulder strap region.
U.S. Pat. No. 5,320,262 discloses an internal frame backpack having a single frame member made of a plurality of rigid strips wherein the frame fits into a single zippered pocket on the backpack. Shoulder straps and a belt are sewn to the pocket and are not directly attached the frame. Consequently, the frame can move within the pocket relative to the user so that the conformation of the frame to the back of the user is not always optimal. In addition, the need for this type of frame to be rigid in a vertical direction makes the frame relatively uncomfortable since it will not flex with the forward and backward rotations of the user's back.